The bubble will float away only if the normal thrust force on the bubble due to the " blown air " exceeds the force on the bubble due to the excess pressure inside it .
Thrust force per unit area - K v 2
Excess pressure - 4 SR
Hence , K v 2 = 4 SR
Or , R = 4 SK v 2
Interestingly , the result doesn't depend on the radius of the pipe , because I made the assumption that , " b >>> R " . If these two are comparable , then this problem would be a very tough one indeed !